feat: the artist picker on Patreon and SubscribeStar too — and the Patreon name is canon (milestone 429)

Operator: "yes add the artist picker to patreon and subscribestar too. but
generally we treat the patreon name as the canon"

- Every add now goes through the panel. On Patreon/SubscribeStar it opens
  on the creator's display name (read only when the panel opens:
  probe?names=1), searches FC's artists with it, and auto-picks a match.
  An untouched URL handle sends no name, so the server still resolves it.
- Patreon is canon: joining a Patreon source to an artist known by another
  name offers "Rename “x” to the Patreon name “X”", ticked by default.
  quick-add's use_platform_name renames server-side from the name it reads
  itself; name only, the slug never moves (#130); never to a URL handle when
  the name can't be read; ignored on SubscribeStar and Discord.
- _platform_display_name returns None rather than the handle, bounded by the
  same 6s lookup budget as Discord's names.
- panelDefaults / addRequest / renameOffer replace the Discord-only helpers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

extension/lib/api.js

@@ -104,16 +104,23 @@ class FabledCuratorAPI {
   // artistId connects the source to an existing artist, artistName to the
   // artist of that name (created if new); with neither the server derives the
   // artist from the URL. A Discord channel always sends one.
-  quickAddSource(url, { artistId = null, artistName = null } = {}) {
+  // usePlatformName: a Patreon source joining an existing artist renames it
+  // to the Patreon display name (Patreon is canon; name only, never the slug).
+  quickAddSource(url, { artistId = null, artistName = null, usePlatformName = false } = {}) {
     const body = { url };
     if (artistId != null) body.artist_id = artistId;
     else if (artistName) body.artist_name = artistName;
+    if (usePlatformName) body.use_platform_name = true;
     return this.request('POST', '/extension/quick-add-source', body);
   }
-  probeSource(url) {
+  probeSource(url, { names = false } = {}) {
     // Read-only existence check. Drives the content-script chip's
-    // color/copy BEFORE the operator clicks Add.
-    const qs = new URLSearchParams({ url }).toString();
+    // color/copy BEFORE the operator clicks Add. `names` also reads the
+    // creator's display name from the platform — the Add panel asks for it,
+    // the chip doesn't, so a plain page view never costs a platform request.
+    const params = { url };
+    if (names) params.names = '1';
+    const qs = new URLSearchParams(params).toString();
     return this.request('GET', `/extension/probe?${qs}`);
   }
   // Latest published extension version on this instance — drives the in-app